This invention relates generally to the measurement of hypochlorous acid, and more particularly to an in-line analyzer for continuously and accurately determining the hypochlorous acid content of chlorinated water.
Disinfection of water in drinking water supplies, swimming pools, industrial process and cooling waters and waste-waters is essential to health and safety, for disinfection serves to destroy pathogens and other organisms present therein. The most widely used method of disinfection is chlorination. This involves continuously feeding either chlorine gas, calcium hypochlorite or sodium hypochlorite into the water at a controlled and regulated rate in a sufficient quantity to impart and to maintain a chlorine residual of specified type and quantity.
The quantity of any selected chlorination agent that must be applied is referred to as the "dosage." This dosage quantity, in turn, includes not only the quantity of the chlorination agent required to impart to the water the specified chlorine residual but also includes the quantity of chlorination agent required to satisfy the chlorine demand of the water. By convention, all three of these distinct quantities--the demand quantity, the residual quantity, and the dosage quantity--are expressed in parts per million (ppm); i.e., parts or chlorine per million parts of water.
The chlorine demand is the ppm chlorine that is required to destroy all harmful bacteria and to react with any oxidizable organic or inorganic chemical substances present in the water. The chlorine demand value is also related to the pH of the water, the temperature of the water, and the particular type of chlorine residual desired.
Chlorine residual is the ppm chlorine present in the water in a form which is either immediately available or potentially available to react with pollutants introduced into the water. Chlorine residual is of two types: free available chlorine residual, the form immediately available for reaction; and combined available chlorine residual, the form potentially available for reaction.
Free available chlorine is created when chlorine gas dissolves in water. In this form, chlorine is extremely reactive and is highly effective as a disinfectant. Combined available chlorine is chlorine which is present in the form of one of the chloramine compounds. This series of compounds results from the reaction of chlorine and ammonia or some nitrogeneous organic compound. In the chloramine form, the reactivity of the chloramine is reduced considerably.
Since free available chlorine is a far more powerful disinfectant than combined available chlorine, it has heretofore been the practice to continuously monitor the free chlorine residual in the water to be sure that adequate disinfection is maintained. Commonly used for this purpose is an in-line analyzer of the type described in the Morrow U.S. Pat. No. 3,959,087, wherein a sample stream of water to be tested is conducted through an amperometric cell having spaced measuring and counter-electrodes. By impressing a voltage across the electrodes causing this cell to operate in a saturation zone, the current flow through the cell becomes a function of the free available chlorine content of the stream being tested and is independent of the combined available chlorine content.
A commercially-available instrument that is capable of discriminating between free available chlorine and combined available chlorine residual to provide a continuous reading of the free chlorine content of chlorinated water is the "Anachlor" Residual Chlorine Analyzer-Transmitter manufactured by Fischer & Porter of Warminster, Pa. This instrument is described in the Fischer & Porter Instruction Bulletin 17B4200, Revision B, published in 1975.
An Anachlor instrument is not, however, capable of discriminating between the hypochlorous acid (HOCl) component of free available chlorine and the other fraction which is hypochlorite ions (OCl.sup.-). The HOCl species is more active as an oxidizing and disinfecting agent than OCl.sup.-, and it therefore becomes desirable to monitor the HOCl component so that by specifically sensing this process variable, one can regulate the dosage so as to maintain optimum disinfection conditions.
To this end, one must be able to select the HOCl component from the free available chlorine residual. One commercially-available instrument which is designed to carry out in situ measurements of HOCl in wter is the "Delta Chlorine Analyzer" manufactured by Delta Scientific Company of Lindenhurst, N.Y. The crucial element in this analyzer is a polarographic membrane electrode that renders the instrument selectively responsive to the chlorine species whereby its primary response is due to HOCl and its smaller secondary responses are to relatively weak chlorine disinfectants such as OCl.sup.- and chloramines.
One of the problems encountered with an instrument of the Delta type is the tendency of the membrane to absorb water by osmosis. This produces osmotic swelling of the HOCl permeable member and results in failure of the sensor when the membrane ruptures. Moreover, since the Delta instrument does not take into account the effect of variations in temperature and pH factor in HOCl, the instrument is unreliable and inaccurate in operation.